The increasing of higher level of integration within electrical integrated circuit (IC) leads to both higher data rates and larger numbers of IC interconnections. Today, the inherent signal speed of IC has been increased to 3 GHz, and shortly it will reach 10 GHz and beyond. The number of pin connections is also increasing, with a single IC requiring close to 2000 interconnections (i.e. single processor), and shortly it will reach over 5000 interconnections. Simultaneously achieving higher data rates and higher interconnect densities for off-chip will be increasingly difficult as the IC technologies continue to evolve, increasing the signal speeds of electronic devices and the number of interconnections. In the off-chip case, high density interconnects, covering from die-level packaging to chip-to-chip (hereafter chip indicates the die with package) interconnection on the PCB, will also be getting increasingly difficult as the IC technologies continue to evolve, increasing the signal speeds and interconnection numbers.
With the increase of signal speeds and interconnection numbers of the IC, it is highly desirable to make low-cost high-speed interconnect techniques compatible with today's manufacturing processes available in consumer level. Today's PCB is mainly made of uniform FR4 material, and their manufacturing technology along with PCB manufacturing are so well matured that, for the long run, all system vendors like to use FR4 based PCB to keep the system cost low. However, FR4 has material characteristics that limit its usage in high speed if conventional interconnection structures are used. The reason is that conventional FR4 has high dielectric loss which mainly limits the bandwidth of interconnects.
In a conventional PCB, as the signal line is either laid on the dielectric material or embedded into the dielectrics, the signal experiences dissipation while propagating through the signal line, based on the dissipation factor (tangent loss) of the dielectric material used as the core layer in the PCB. The reason is that the electric field starts from the signal line and ends in the ground, and this electric field passes through the dielectric. This signal dispersion is proportional to the signal frequency, i.e. signal speed. This means that the higher the signal speed, the lower the distance of transmission of signal for the fixed dielectric material. In the other words, the higher the speed, the lower the bandwidth of the signal line which is used for connecting one chip to another chip on the board. If the tangent loss of the dielectrics are high, the bandwidth of the interconnects gets so limited that high speed signals cannot be sent over longer distances as compared with dielectrics having lower tangent loss.
In addition to tangent loss, the dielectric constant of dielectric material is also important, as an electrical field inside dielectric material having a higher dielectric constant experiences more signal delay as compared with that of a transmission line comprising lower dielectric constant material. This causes signal skews for different length signal lines. In this case also, lower dielectric constant material is necessary in the interconnection for high-speed signal interconnection. This is true for both on-chip and off-chip interconnection. Lower dielectric constant material with low dielectric loss offers the following functions: (1) higher density interconnection is possible due to reduction of the cross talk, (2) reduction of the capacitance of the interconnection, helping to transfer the signal longer distances, and (3) lower propagation delay.
Considering signal loss and signal delay for various signal line lengths, it is highly desirable to design the interconnects on PCB with lower effective dielectric constant and lower effective loss of the interconnect system.
It is very straight forward that increasing the bandwidth can be possible by the usage of material having lower loss tangent (dielectric loss). However, in this case, for off-chip interconnection new material development is necessary. Besides, manufacturing technology needs to be developed to implement at the product level. Conventionally, to increase the interconnects bandwidth, dielectrics having lower tangent loss are used as the PCB layer. This dielectric material is very high cost, and the manufacturing processes for building PCB using these materials have not matured yet. In addition, the PCB made of such low loss material has low reliability. It is highly desirable to have high speed PCB that can be built up with the conventional well-matured dielectric material (for example FR4) and also conventional well-matured fabrication processes. This can not only reduce the cost, but also have high reliability.
Much work can be found in off-chip interconnection technology focusing on material development. As for example, low loss materials like Rogers R/flex 1100, etc. are at the development stage to achieve high bandwidth. Implementing new material in PCB fabrication processes will cost tremendously to make it mature. In addition, new materials having low tangent loss are incompatible with conventional dielectric materials, such as FR4 processing, and thus is not a low cost solution. These materials will require a much higher temperature and pressure for bonding. Today, in developing the high speed PCB, more focus is on shortening the length or on the interconnection layout. In both cases, implementing technology would come at high cost.
As explained above, the conventional PCB technology being used for off-chip interconnection cannot be used as the need of the signal speed is increasing. Also, existing conventional electrical interconnects have the limitation of achieving the bandwidth in certain level, beyond which complete manufacturing technology is needed to be changed which is costly for the PCB industry. It is highly desirable to have lower dielectric constant and lower dielectric loss (loss tangent) by adopting a technique or method which can be easily implemented, and which can use the standard dielectric material PCB technology.
In addition off-chip (on PCB) interconnects, or on-chip (inside IC) are also getting very critical issues as both frequency and interconnect density are increasing. It is estimated that in today's chip, more than 50% of total on-chip's power requirement is due to interconnects related loss. Reducing the power loss in interconnects of on-chip can able to reduce significantly the IC power requirement.